More and more applications are moving to the cloud: Google Docs for productivity apps, Meebo for instant messaging, Gmail for e-mails, Salesforce for CRM, etc.
Yet, I\'
I see two reasons. First of all, people might have javascript turned off, today that might not be the biggest issue of all but it's been following developers since the dawn of web-dev. Which brings us to the second reason, it's not what the users expect.
If you were to talk to someone about usability on the web and you had "hidden" features which were only made visible on right-click(browser context menu popup), this would probably be counted as bad design just because most users aren't use to the idea that the web is evolving into something more than just links and left-clicks(navigation clicking).